Optical imaging systems

ABSTRACT

A substantially unity magnification image forming system includes at least one convex mirror and at least one concave mirror supported with their centers of curvature substantially coincident and means to define a location for an oject the image of which after at least three reflections including at least one reflection at each of said mirrors is a real image at a second location and a monocentric meniscus lens between the mirrors to give overall correction for Petzval sum.

This invention relates to optical imaging systems and, in particular, tooptical imaging systems for forming an image of an object at unitmagnification.

U.S. Pat. No. 3,748,015 describes an optical imaging system for formingan image of an object at unit magnification and high resolution by meansof convex and concave spherical mirrors arranged with their centres ofcurvature coincident. The mirrors are arranged to produce at least threereflections within the system and they are used in the system. Twooff-axis conjugate areas at unit magnification are in a plane whichcontains the centre of curvature, the axis of the system being an axisnormal to this plane and through the mirror centre.

Any unit magnification system that consists of two equal halves disposedsymmetrically about a central aperture stop is intrinsically free fromvarious types of aberration, in particular all orders of distortion andcoma. The prior art system shares this characteristic, but it suffersfrom other aberrations and limitations which restrict its performance.In particular:

(a) It is inherent in the design that it only gives sharp imagery over aquite narrow annular area in the focal plane, so that it is necessary touse the system with a narrow arcuate slit aperture exposing only thisarea, and to copy the object (mask) to an image surface by scanning themboth, in synchronism, across this aperture. This requires extrememechanical precision if one micron or less is to be resolved; and thescanning process is obviously slower than direct exposure over the wholearea.

(b) For any aberration free optical system, there is an inherent limitof resolving power, which is proportional to the numerical aperture(N.A.) and in the prior art system there is a practical limit to theattainable N.A. In the specific embodiment of the prior art (and inpractical realisations thereof) this maximum N.A. is about 0.18(equivalent at f/2.78) which gives a theoretical limit of resolution (atwhich the optical transfer function falls to zero), at a wavelength of400 nm, at a spacing of 1.1 μm. This is certainly inadequate for futurerequirements. A larger N.A. could only be realised by moving object andimage further from the centre, and as this is done the aberrationsincrease, and the masking slit width decreases rather rapidly.

In order to overcome this drawback, we have devised a means of improvingthe correction of aberrations.

In Sov.J.Opt.Technol. 50(3), March 1983 153, there are described unitmagnification imaging systems with compensation meniscus lenses.However, the design parameters of the systems described therein areconstrained so that the monocentric meniscus is wholly on the objectside of the secondary mirror so that it is not traversed a second timeby light between the first and second mirror reflections. We have foundthat, contrary to this teaching, far greater improvement in imageryarise when the meniscus lies between the mirrors.

According to the present invention, there is provided a substantiallyunity magnification image forming system comprising at least one convexmirror and at least one concave mirror, said mirrors being supportedwith their centres of curvature substantially coincident and means todefine a location for an object the image of which after at least threereflections including at least one reflection at each of said mirrors isa real image at a second location and further comprising a monocentricmeniscus lens between said concave and said convex mirrors substantiallyto give overall correction for Petzval sum, to produce with saidmeniscus lens at said second location a stigmatic image of an object atsaid first location.

The invention will now be described, by way of example, with referenceto the accompanying drawings, in which;

FIG. 1 is a diagrammatic representation of a prior art catoptric imagingsystem

FIGS. 2, 3, 4a and 4b are diagrams used in explaining the operation ofthe imaging system of FIG. 1

FIGS. 5a-b are diagrammatic representations of a second prior artimaging system; and

FIGS. 6a-c are diagrammatic representations of imaging systems inaccordance with the present invention.

Referring now to FIG. 1 of the drawings, an imaging system comprises twospherical mirrors, a convex mirror 1 and a concave mirror 2, arranged toprovide three reflections in the system. The mirrors are arranged withtheir centres of curvature coincident and to have off-axis conjugateareas centred at the points O and I. The points O and I are each at adistance h from the reference axis A and at opposite sides thereof.

Although the object and image surfaces are shown as coplanar, they canbe separated to more convenient locations by the insertion of planemirrors, but this does not affect the mechanical precision needed forscanning.

For such a system, we consider a ray of light, undergoing threereflections. The line joining the common centre of the mirror P, to thepoint where the ray meets the convex mirror obviously constitutes anaxis of symmetry, and the incident and emergent rays are equidistantfrom the common centre on opposite sides of the axis, and theirinclinations with the axis are equal and opposite (FIG. 2). We take anobject/image plane through P, normal to this axis, containing object andimage points O and I, and consider this first ray as the central ray(principal ray) of a pencil emerging from O.

An aperture ray of this pencil, shown dotted in FIG. 3, makes anaperture angle u with the principal ray. Since the system ismonocentric, this aperture ray, after three reflections, will emerge atthe same distance as the incident ray from P, on the opposite side, andequally and oppositely inclined to the line joining P to the point ofincidence of this ray on the convex mirror (a subsidiary axis).

The emergent aperture ray will not pass through the image point P (theimagery will be aberrated) unless the incident and triply reflected raysare parallel. When this condition is satisfied, the system is said to betelecentric, and the simple monocentric three-reflection system isinherently incapable of meeting this condition over an extended range ofobject/image positions. The system is said to show spherical aberrationof principal rays.

The condition is approximately satisfied for an object very close to thecentre of curvature, if the Petzval sum is zero (for this system, thismeans that the radius of the convex mirror is one half that of theconcave). but in that case principal rays from objects further away fromthe mirror centre, that strike the centre of the convex mirror, becomenecessarily more divergent (FIG. 4a). By a suitable choice of mirrorradii, principal rays at some required object distance can be madetelecentric. but at larger and smaller object distances, they will bedivergent or convergent respectively (FIG. 4b).

There is no obvious way of securing perfect correction of principal rayspherical aberration in a monocentric optical system but a high level ofcorrection may be obtained by adding an appropriate monocentric meniscuslens to the three-reflection system of the above type, the lens andmirror dimensions being chosen to give approximate overall correctionfor Petzval sum.

The level of improvement depends on the location of the meniscuscorrector. First order correction of principal ray spherical aberrationcan be provided by a meniscus whose concave surface is relatively closeto the focal plane. in which case the meniscus will be relatively thin(FIG. 5a) or by a meniscus further from the focal plane (and thereforehaving shallower curvatures) in which case the meniscus must be thicker(FIG. 5b). In each case correction of primary principal ray sphericalaberration is possible, but perfect correction is not obtained due tothe presence of higher orders of aberration although these uncorrectedresiduals are successively reduced, as the meniscus corrector is movedaway from the focal plane, and the overall level of aberrationcorrection of the system is thereby improved. We have found that asignificant improvements is obtained when the meniscus lens ispositioned between the two mirrors.

The effect of this is that the limitations of performance of the priorart system discussed above are considerably reduced. The narrow width ofthe annular area of sharp imagery is greatly extended, for example, alevel of image quality in the prior art arrangement which restricts theslit width to 0.25 mm can permit a width of a centimeter or more. And itbecomes possible to secure good performance over larger numericalapertures than the simple 3-mirror system can yield, for example 0.28 inplace of 0.18.

Fabrication may be simplified in the preferred embodiments where one ofthe mirrors is deposited as a reflecting layer on part of a surface ofthe meniscus.

EXAMPLE 1

A 1:1 copier lens of numerical aperture 0.28 has a construction as shownin FIG. 6a. It is corrected for object/image distances from the centralaxis 5.6 cm to 5.9 cm (slit width 3 mm). Its parameters are as shown inTable 1.

                  TABLE 1                                                         ______________________________________                                                        axial        refractive                                       surface         separation   index   full                                     number                                                                              radius    10.0 to object                                                                             1.0 (air)                                                                             diameter                                 ______________________________________                                        1     -10.0000                       15.6                                                     2.6220        1.5010                                          2     -12.6220                       18.2                                                     10.4876       1.0                                             3 (M1)                                                                              -23.1096                       25.6                                                     -10.4876     -1.0                                             4     -12.6220                                                                                -2.6220      -1.5010                                          5 (M2)                                                                              -10.0                          5.6                                                      2.6220        1.5010                                          6     -12.6220                                                                                10.4876       1.0                                             7 (M3)                                                                              -23.1096                       25.6                                                     -10.4876     -1.0                                             8     -12.6220                       18.2                                                     -2.6220      -1.5010                                          9     -10.0000                       15.6                                                     -10.00 to focus                                                                            -1.0                                             10    ∞ image                  11.8                                     ______________________________________                                    

Dimensions in centimeters. A negative radius is concave to theobject/image plane. A change of sign of refractive index indicates areflection.

EXAMPLE 2

A 1:1 copier lens of numeric aperture 0.23 has a construction as shownin FIG. 6b. It is corrected for object/image distances from the centralaxis 5.5 cm to 6.0 cm (slit width 5.0 mm). Its parameters are as shownin Table 2.

                  TABLE 2                                                         ______________________________________                                                        axial        refractive                                       surface         separation   index   full                                     number                                                                              radius    13.0 to object                                                                             1.0 (air)                                                                             diameter                                 ______________________________________                                        1     -13.0000                       16.7                                                     24.9032       1.5000                                          2 (M2)                                                                              -37.9032                       36.2                                                     -24.9032     -1.5000                                          3     -13.0                                                                                   -1.4344      -1.0                                             4 (M2)                                                                              -11.5656                       5.2                                                      1.4344        1.0                                             5     -13.0                                                                                   24.9032      -1.5000                                          6 (M3)                                                                              -37.9032                       36.2                                                     -24.9032     -1.5000                                          7     -13.0                          16.7                                                     -13.0000     -1.0000                                          8     ∞ image                  12.0                                     ______________________________________                                    

Dimensions in centimeters. A negative radius is concave to theobject/image plane. A change of sign of refractive index indicates areflection.

The possibility of various modifications which remain within the ambitof the invention will be apparent to those skilled in the art. Forexample, although single concave and convex mirrors have beenillustrated, it is possible to use a plurality of mirrors of the equalor different curvatures to achieve the same effect.

I claim:
 1. A substantially unity magnification image forming systemcomprising at least one convex mirror and at least one concave mirror,said mirrors being supported with their centres of curvaturesubstantially coincident and means to define a location for an objectthe image of which after at least three reflections including at leastone reflection at each of said mirrors is a real image at a secondlocation characterised in that it comprises a monocentric meniscus lensbetween said concave and said convex mirrors substantially to giveoverall correction for Petzval sum, to produce with said meniscus lensat said second location a stigmatic image of an object at said firstlocation.
 2. A substantially unity magnification image forming system asclaimed in claim 1 characterised in that at least one of said mirrors iscontiguous with a surface of said meniscus lens.
 3. A substantiallyunity magnification image forming system as claimed in claim 2characterised in that said concave mirror is formed on a surface of saidmeniscus lens.
 4. A substantially unity magnification image formingsystem as claimed in claim 2 characterised in that said convex mirror isformed on a surface of said meniscus lens.
 5. A substantially unitmagnification image system as claimed in claim 1 characterised in thatit includes a plurality of concave mirrors.